First off that's a really subjective question so it's REALLY hard to give a definitive answer, but...my guess is that you'll struggle at high/ultra. The 6850 isn't the worlds best performer and your CPU will show its age in what amounts to a very CPU intensive environment.
Doubtful unless you drop down the settings a good bit. BF3 is a pretty graphic intensive game and the 6850 is kinda puny when it comes to 1920x1080. A pair of them would give a lot better chance of hitting the frame rates you want. Upgrading to a 6 core processor would give you a lot better chance of hitting that as well because a 50 player server is a pretty big load for a CPU and the more threads the better.
The answer to your question is no. 6850 is not a high end card to average even 40FPS at 1080p your likely going to be at low settings maybe some medium with no extra eyecandy. Your CPU is a slight bottleneck too but that can be fixed by OC'ing if your on the stock cooler try to take it to 3.7GHz (980BE speeds) if you have a better cooler shoot for 4GHz pair that with a much better GPU and you can get over 60FPS at decent settings.
Heres an idea of how CPU's compare if you match them with a higher GPU on a 64 player map